1. Field of the Invention
This invention relates to flame retardant ethylene-alkoxy silane copolymer insulation compositions which exhibit good processability and are capable of being cured under ambient conditions.
2. Description of the Prior Art
Crosslinked ethylene polymers are used extensively for wire and cable insulation. Crosslinking improves the mechanical strength, heat resistance and other properties required for most insulation applications.
One type of widely used crosslinkable flame retardant ethylene polymer insulation is comprised of ethylene-vinyl acetate (EVA) copolymer, hydrated organic filler, silane compound and a chemical crosslinking agent, most commonly an organic peroxide. Compositions of this type are disclosed in U.S. Pat. Nos. 3,832,326; 3,922,442; 4,349,605; and 4,381,362. Due to the presence of the chemical crosslinking agent during processing and extrusion, operating conditions are limited and must be carefully controlled to avoid premature crosslinking, commonly referred to as "scorching." Premature crosslinking results in poor extrudate quality. To minimize scorching processors often must use conditions which require them to operate at line speeds below the maximum capabilities of the equipment.
To develop optimal physical properties with the EVA insulations, the coated wire or cable product must be subjected to a fairly rigorous curing operation to crosslink the composition. Curing is accomplished using a continuous vulcanization (CV) line where the extruded wire or cable product is contacted with steam at approximately 400.degree. F. and 200 psi. CV lines are expensive to maintain and operate.
Moisture curable compositions have been developed which eliminate the need for steam curing. These compositions utilize ethylene polymers which have alkoxy silane functionality incorporated into the polymer either by grafting an unsaturated alkoxy silane onto an ethylene polymer or by directly copolymerizing ethylene with an unsaturated alkoxy silane. In the presence of moisture the alkoxy silane groups undergo hydrolysis and condensation reactions to form crosslinks. Silanol condensation catalysts are typically used to increase the rates of hydrolysis and condensation.
Crosslinkable ethylene polymers prepared by grafting unsaturated alkoxy silanes to ethylene polymers are disclosed in U.S. Pat. No. 3,646,155. Crosslinkable ethylene polymers prepared by copolymerizing ethylene with an unsaturated alkoxy silane are disclosed in U.S. Pat. Nos. 3,225,018 and 3,392,156.
Ethylene-alkoxy silane copolymers can also undergo premature crosslinking during processing. Premature crosslinking adversely affects (lowers) the melt index of the resin and, if this occurs to a significant degree, will result in poor processability and unsatisfactory extrudate surface appearance. In extreme cases, extrusion of the ethylene-alkoxy silane copolymer compositions becomes essentially impossible. Therefore, when considering any modifications to silane copolymer formulations for the purpose of improving properties of the resulting insulated product, careful consideration must be given to the effect on processability.
Processing problems are particularly troublesome with formulations which contain a silanol condensation catalyst and with filled compositions since particulate fillers can obtain substantial amounts of moisture. This moisture is released during the mixing and blending operations and hydrolyzes some of the alkoxy groups of the alkoxy silane. The processing problems associated with the use of fillers with ethylene-vinylalkoxy silane copolymers are well documented and discussed in detail in European Patent Application 89310667.4 published Apr. 25, 1990.
U.S. Pat. No. 4,397,981 discloses the use of organic halogen compounds, such as chlorinated polyethylene, to impart flame retardance to moisture curable random copolymers of olefins and olefinically unsaturated silane compounds. The reference states that undesirable premature crosslinking is obtained when certain halogenated organic compounds are employed. This adversely affects processability and results in extrudates with undesirable surface blemishes.
It would be highly useful if flame retardant olefin-alkoxy silane copolymer compositions were available which exhibited good processability and improved cure rates. It would be even more advantageous if these flame retardant compositions could be acceptably cured under ambient conditions, i.e., without exposure to water baths or steam autoclaves, and if the resulting ambient cured product met all of the specifications of SAE J-1128 for thin wall crosslinked polyolefin insulated low tension primary cable. These and other advantages are realized with the compositions of the present invention which are defined in more detail to follow.